Zinc plating thickness vs. salt spray resistance

A. Hi Mihir. No, a higher carbon steel will not deter the rusting, and will only make the plating somewhat more difficult.

Repeating from an earlier comment on the thread which we added your inquiry to: You will pass the 96 hour salt spray test "if the substrate was properly mechanically prepared; and it was properly cleaned and activated; and it was properly zinc plated; and it was properly trivalent chromated with a proprietary chromate and the topcoat recommended by the supplier; and the shape of the component is such that it can get proper plating coverage, and the process is under control."

Luck and Regards,

? Hi,

Can you tell us what type of zinc solution you use? Is it acid, cyanide or alkaline? Also the chromate you use. This will help to give you a answer on your question.

Regards

A. Hi Deepak. Although 96 hours would be a good answer, remember that salt spray testing is for the purpose of keeping the process under control, not predicting real-world life. So, what is your evidence that 12 microns would be enough if the process were under good control?

In other words, are you sure that the plated parts which rusted are seeing the same exposure conditions as the parts which didn't? Are you sure that your exposure conditions are not "very severe" (requiring 25 microns of plating)? Is there any exposure to road salt, lawn fertilizer, pressure treated wood, or other chemicals? Is the problem red rust or white rust? Thanks!

Regards,

A. Hi again, Deepak. Due to the very close timing, it's not clear to me whether your reply was in response to my posting or just coincided with it. But I think the ball is still in your court :-)

Regards,

A. Hi Jayant. If you read literature from 30 years ago, it will say that hexavalent is much better, which it was. But today's modern proprietary trivalent chromates are the equal of hexavalent chromates (in salt spray testing at least).

Regards,

A. Hi Andrew. For my own part, in a career of 50+ years in electroplating, I don't recall even once seeing zinc electroplating which wasn't chromated, but Tom Rochester suggests that you look at ASTM B695 for that answer. Good luck.

Regards,

A. Hi Ansh. I think the failed thickness check rather than the failed salt spray test is the heart of this matter!

If the screws are supposed to be plated to 12 micron thickness, but the supplier is not plating them for a long enough time to achieve that requirement, something is terribly wrong and must be fixed before you can proceed. The vendor perhaps justifies themself by believing that your spec is incorrect or poorly written ... and they may be right or wrong. But whether they agree to plate to your spec or you agree to change the spec, STEP 1 is that the plating thickness must conform to the spec; if it doesn't, everything else is nothing but nonsense :-)

Regards,

A. Hi Ansh. As Ted said, first verify why the bolts have less thickness than specified. It could be that your supplier didn't know the spec (it wouldn't be my first), mistakes in plating time or current, low zinc/low anode surface, low bath conductivity (potassium/ammonia?)...

Not knowing your supplier, you can make many assumptions, but what you have is a part not meeting your spec (12 microns minimum, 264 hour of SST without corrosion of base metal). Go with that and listen to what they say :)

Best of luck!

A. Hi Pankaj. I doubt it. But at the least, 480 hours sounds equivalent to very severe exposure (25 microns) rather than moderate (8 microns) exposure. Can you tell us what spec you intend to evaluate the salt spray life according to, and give us some background info? Thanks.

Regards,

A. Hi again Pankaj. Thanks for the additional info. The other people on this thread have been discussing zinc plating; zinc-nickel plating is a different process and is much more corrosion resistant. On topic 28851, Asif Nurie asserts that 500-1000 hours is possible, but some other posters reported failure within that time frame. The way to confirm the ability to comply is to plate sample parts and test them for 480 hours, then evaluate them in accord with some specific specification.

The problem remains that the results will depend not only on the plating process but on the shape of the part, its substrate material, its surface finish, etc. To "teach to the test", designing to salt spray hours instead of real life is a mistake; but to try to do so while ignoring the actual parts is impossible. Good luck.

Regards,

A. Probably zero, cousin Umesh, because it will probably not accept chromate properly, and may rust in the time before & after salt spray exposure. I've never heard of anyone trying to use only 1 micron of zinc plating. 5 microns is specified for "mild" service conditions.

Regards,

A. Hi Umesh,

I'm not sure what life 1 micron will achieve but your business life expectancy with your customer will be zilch if he finds out you were asking..  :)

Yay Khozem!

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Andy. Yes! .0002" (5 microns) is for "mild" service per ASTM B633 and related specs, and .0005" (13 microns) is for "severe" service. In between is.0003" (8 microns) for "moderate" service.

Regards,

A. Hi Lawrence. If you can plate them, you should be able to get 200 hours with a very good chromate conversion coating. But I think you might want "clear" rather than "blue", and I'm not sure you will be able to plate oil quenched parts unless you have a very good cleaning cycle. Further, this application sounds like "very severe" service", such that zinc plating thickness should be specified rather than being satisfied simply with a white rust salt spray test. Good luck.

Regards,

A. Hi Dhanaj. Per ASTM B633 and some related industry standards, "severe" service should have 13 microns, 'very severe' (bold atmospheres, cleaners, saline solutions, denting, scratching and abrasion) 25 microns. It seems to me that automotive and some others tend to use alloy platings like zinc-nickel of the next thicker thickness instead (8 microns of alloy plating for the severe stuff and 5 microns for the moderate).

192 or 200 hours to white rust seems to be pretty typical these days. As for red rust, I'd concentrate on the thickness rather than the hours to red rust since salt spray testing tells us little to nothing about real-life endurance of zinc coatings because the corrosion mechanism is fundamentally different.

Regards,

A. Hi Sateesh. You are asking an extremely broad abstract question which is probably more appropriate for a book, or at least a chapter, rather than a brief public forum response :-)

The short answer is, no, the salt spray resistance is generally not directly proportional to the thickness. In zinc and zinc alloy plating the quality of the chromate conversion coating is probably a bigger factor in salt spray resistance than the thickness of the zinc plating. But zinc is anodic to iron so there is at least some degree of proportionality between plating thickness and salt spray hours because the zinc plating will gradually dissolve.

But in other types of plating, where the plating layer is cathodic to the substrate, such as nickel plating on steel, the 'porosity' of the plating is far more important than the thickness because corrosion begins as soon as the salt solution reaches the steel substrate. As a general rule, increasing the thickness of the plating probably decreases the porosity, but a properly applied fairly thin plating layer may be less porous and pin-holed than a poorly applied thicker layer.

Please introduce yourself and your particular situation; people can often help you with a fairly specific problem but, much as they may wish to, they can't condense decades of experience into a few paragraphs for you :-)

Regards,

A. Hi Jamal. What company or international standards body are you referring to with that spec? Different automobile companies have specs that may use similar abbreviations and which look sort of the same but which are not the same specs. In some specs the number after MFZn is the thickness in microns, but I doubt that that's the case here; it's perhaps a Honda spec where the "2" means exposure class 2, not 2 microns.

Under ideal conditions, yes, a well controlled zinc plating process can probably deliver 240 hours, quite possibly more. Does the specification mention 240 hours or is that only mentioned on the drawing? Thanks.

Regards,

A. Hi Jason. I think you should try to track one batch of parts from the receiving dock to the salt spray cabinet. Very often the flow sheets, no matter how carefully written, don't describe what really happens. "Walking the line" often immediately reveals what is wrong ("No good decision was ever made from a swivel chair" -- General George S. Patton).

Some thoughts: The spec probably calls for no "white rust" until 120 hours, let alone "red rust". The high salt spray hours bragged about by the process vendors always apply to ideal shaped parts and after excellent mechanical preparation. Has the plater made any comments about "design for plating"? Are the hinges properly deburred? Are these "hinges" pre-assembled, such that capillary action may mean that acids are drawn in from early process steps are carried through the process? Is there a particular area on the hinge which rusts first? Has the plater recently changed chromate suppliers?

Regards,

A. Hi Raymond. SC 3 means Service Condition 3, "Severe Service", and requires 13 microns of plating thickness. Please never quote to plate to a spec without the spec in hand :-)

Regards,

A. Hi Guhan. You ask "Can we accept it?" but haven't introduced yourself as purchaser, test lab, or plating shop

What usually actually answers most questions is the plating specification. Someone should have, and possibly did, write or reference a specific plating specification for the plating contract. Please give us that specification or specification number and we can probably point to a specific line which defines such plating as unsatisfactory. Good luck!

Luck & Regards!

A. Hi Guhan. The part is probably satisfactory; you are almost surely not supposed to recheck minimum thickness after an accelerated corrosion test, but again we must ask for a specification or specification number before anyone can say for sure whether the part is satisfactory. For example, you say "without any rusting", but do you mean red rust or are you including white rust ... and which one does the spec say you must resist for 96 hours? Salt water isn't terribly abrasive so it's hard to imagine losing 1.8 µm of plating without generating any white rust. And does the spec actually say that testing one sample part is enough, or might it demand a specific sampling plan? You cannot assert that the quality meets the requirements without a specification number telling you what those quality requirements are.

If you received just a vague requirement like "8 µ Zn + yellow chromate, must pass 96 hour salt spray", then you should pick a spec or write a spec that answers such questions because without it you cannot have quality assurance. For example, what happens if you're out sick next week and your replacement views things differently?

Luck & Regards,

A. Hi Gopinath. That does seem like outstanding performance, but I guess the answer is that you did a good job of applying a very good trivalent chromate and sealer onto a very good zinc plating layer on a very well prepared substrate. But as for "only 15 microns" that is not thin zinc plating, it's more than enough for 'severe' service.

If you are implying a suspicion that the salt spray test was conducted incorrectly, please try to be a bit more explicit and specific. Thanks!

Luck & Regards,

A. Hi Venkat. I would suggest that you read this whole thread because several points have been made repeatedly here ...
1. Although Tom Rochester notes below that ASTM B695 for mechanical plating has a table of thickness vs. salt spray hours, I don't think you should be selecting the thickness based on desired salt spray results, but based on the environment. If the environment is 'harsh', that sounds like what the specs call 'severe service', and the thickness should be 13 microns.
2. What standard will be used to evaluate whether the parts passed? And does it call for no red rust or for no white rust?
3. When looking for 500 hours salt spray, you might want to follow the example of the automotive companies and specify zinc alloy plating (zinc-iron, zinc-cobalt, zinc-nickel, or tin-zinc) rather than plain zinc. The thicker an electroplating is, the less ductile it tends to be; so for use on springs, a thinner zinc alloy electroplate may be preferable to a thicker zinc electroplate. But considering that steel springs are made of hardened materials subject to hydrogen embrittlement, you might also consider mechanical plating in lieu of electroplating.

Sorry, I am not familiar with attempting to apply a home-brew generic top coat onto trivalent chromate. In my experience, the top coat has always been purchased from the trivalent chromate supplier.

Luck & Regards,

A. A trivalent conversion requires a sealer. This is a must.

A. Hi Santosh. What plating specification are you attempting to adhere to? What process are you using? I presume it's barrel plating, but cyanide, acid, or alkaline non-cyanide. What chromate are you applying ... is it a proprietary trivalent with proprietary topcoat?

Luck & Regards,

A. Hi ANUBHAV. The 7µ minimum thickness specified should get you the salt spray hours you are looking for if the trivalent chromate and top coat are high quality, and assuming that the entire process is well controlled.
Proper thickness is supposed to be determined by real world exposure conditions; for example, according to ASTM B633: 5µ for mild exposure, 8µ for moderate, and 12-13µ for severe.
The purpose of salt spray testing of zinc plating is to give you a quick heads-up if you are losing control of the process; you are supposed to design parts for real world exposure conditions not to achieve salt-spray hours.

Luck & Regards,

A. Hi Sathyaraj. Apparently Hyundai thinks so :-)

I don't mean that cynically or satirically, but rather that they apparently feel that if the metal has received proper mechanical pretreatment and proper chemical pretreatment, and the zinc plating, chromating, and post-treatment are well controlled it is possible with that zinc plating thickness. Of course cyanide zinc and alkaline non-cyanide zinc give more uniform thickness than acid zinc and you may have thin spots.

Luck & Regards,

A. Hi Ammar. Cyanide zinc and alkaline non-cyanide zinc offer better covering power and throwing power (evenness of plating thickness) than acid zinc, so there should be less problem with uncovered or minimally covered areas, and theoretically better salt spray performance. But realistically, 96 hours should not be difficult for any modern zinc plating and chromate conversion coating; 96 hours tests the chromate more than the zinc plating.

We've said a thousand times that design decisions should not be based on optimizing salt spray time; instead, salt spray testing is an early warning that a process is going out of control. I guess as long as people keep saying that that is what they want to do, we'll have to keep repeating that it's not a good idea :-)
As one obvious example, if we make decisions about choice of finish based on salt spray hours we'd never pick hot dip galvanizing; it's expensive, and a much cheaper good paint job will beat it in salt spray hours. But in actuality there is no other finish which even begins to approach hot dip galvanizing in real-world longevity; when away from the sea, three-quarters of a century or even a full century without any maintenance is not unheard of.

Luck & Regards,

A. Hi Ammar. I understand what is common; if someone else in the supply chain has told you to focus your process line toward maximizing salt spray hours rather than actual performance, then my message was directed to them :-)
In any case, as previously mentioned, 96 hours should not be any challenge for a properly operating line; it has been the minimum standard for more than 60 years, and most process vendors claim far more.

These days trivalent chromate conversion coating should probably be done, and that involves not just the chromate conversion step, but a sealing step. Usually the immersion time is not critical, but is in the general range of 45 seconds to twice that; the process supplier will offer a technical data sheet specifying the optimum and acceptable range for immersion time.

Hexavalent chromate drying temperature should not exceed 140 °F (60 °C); I have heard that a bit higher is okay for trivalent chromates, but why go higher if you don't have to.

Luck & Regards,

A. Hi Gaurav,
We appended your inquiry to a thread where the question has been answered quite exhaustively. However, I would add the following in answer to your specific question:
• You should be plating to a specification not just a thickness. Pick a spec, ASTM B695 or ISO 2081 for example, get it and see if it clearly specifies the finish you want.
• I think you will find that even the minimum thickness in the spec, probably 5 µm, will deliver the salt spray hours you need if the substrate is properly pretreated, properly plated, and properly chromated with a high-quality proprietary chromate.
• I believe you'll also find that you are supposed to select the zinc thickness based on service condition, not salt spray hours. 5 µm may pass the salt spray test but that thickness is for mild exposure conditions and 8 µm is more appropriate for auto parts.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey